The corrosion behaviors of Mg-6 wt.%Gd-3 wt.%Y-0.4 wt.%Zr (GW63K) alloy in 5 wt.% NaCl aqueous solutions have been investigated in both as-cast and T6 heat treatment conditions by immersion test and electrochemical techniques. The corrosion products and their morphologies formed in the solution after immersing various time have been analyzed by FE-SEM and XRD. It was found that the morphologies of the film of corrosion products varied with immersion time and changed from honeycomb into porous structure composed of tiny erect flaks after immersing 60 min. The phenomena of undermining and falling away of the noble second phase particles observed in the test can be used as an evidence to support the Mechanism II explaining the mechanism of negative difference effect (NDE) about magnesium and its alloys. It was indicated that the E-CORR of GW63K alloy in T6 condition (-1.71 V) is little nobler than that in as-cast (-1.73 V). It was shown by pitting scans that GW63K alloy has tendency to pit whether in as-cast or in T6 condition and the tendency in T6 condition is more serious than that in as-east. An equivalent circuit was proposed based on the EIS experimental data and the honeycomb or porous characteristics of the corrosion products formed on the surface of GW63K alloy. (c) 2006 Elsevier Ltd. All rights reserved.